12.12.7 Formatted Output Functions

This section describes how to call printf and related functions. Prototypes for these functions are in the header file stdio.h. Because these functions take a variable number of arguments, you must declare prototypes for them before using them. Of course, the easiest way to make sure you have all the right prototypes is to just include stdio.h.

Function: int printf (const char *template, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe mem lock corrupt | See POSIX Safety Concepts.

The printf function prints the optional arguments under the control of the template string template to the stream stdout. It returns the number of characters printed, or a negative value if there was an output error.

Function: int wprintf (const wchar_t *template, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe mem lock corrupt | See POSIX Safety Concepts.

The wprintf function prints the optional arguments under the control of the wide template string template to the stream stdout. It returns the number of wide characters printed, or a negative value if there was an output error.

Function: int fprintf (FILE *stream, const char *template, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe mem lock corrupt | See POSIX Safety Concepts.

This function is just like printf, except that the output is written to the stream stream instead of stdout.

Function: int fwprintf (FILE *stream, const wchar_t *template, …)

Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe mem lock corrupt | See POSIX Safety Concepts.

This function is just like wprintf, except that the output is written to the stream stream instead of stdout.

Function: int sprintf (char *s, const char *template, …)

Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

This is like printf, except that the output is stored in the character array s instead of written to a stream. A null character is written to mark the end of the string.

The sprintf function returns the number of characters stored in the array s, not including the terminating null character.

The behavior of this function is undefined if copying takes place between objects that overlap—for example, if s is also given as an argument to be printed under control of the ‘%s’ conversion. See Copying and Concatenation.

Warning: The sprintf function can be dangerous because it can potentially output more characters than can fit in the allocation size of the string s. Remember that the field width given in a conversion specification is only a minimum value.

To avoid this problem, you can use snprintf or asprintf, described below.

Function: int swprintf (wchar_t *s, size_t size, const wchar_t *template, …)

Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

This is like wprintf, except that the output is stored in the wide character array ws instead of written to a stream. A null wide character is written to mark the end of the string. The size argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least size wide characters for the string ws.

The return value is the number of characters generated for the given input, excluding the trailing null. If not all output fits into the provided buffer a negative value is returned. You should try again with a bigger output string. Note: this is different from how snprintf handles this situation.

Note that the corresponding narrow stream function takes fewer parameters. swprintf in fact corresponds to the snprintf function. Since the sprintf function can be dangerous and should be avoided the ISO C committee refused to make the same mistake again and decided to not define a function exactly corresponding to sprintf.

Function: int snprintf (char *s, size_t size, const char *template, …)

Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The snprintf function is similar to sprintf, except that the size argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least size characters for the string s. If size is zero, nothing, not even the null byte, shall be written and s may be a null pointer.

The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to size, not all characters from the result have been stored in s. You should try again with a bigger output string. Here is an example of doing this:

/* Construct a message describing the value of a variable
   whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
  /* Guess we need no more than 100 chars of space. */
  int size = 100;
  char *buffer = (char *) xmalloc (size);
  int nchars;

  if (buffer == NULL)
    return NULL;

 /* Try to print in the allocated space. */
  nchars = snprintf (buffer, size, "value of %s is %s",
		     name, value);

  if (nchars >= size)
    {
      /* Reallocate buffer now that we know
	 how much space is needed. */
      size = nchars + 1;
      buffer = (char *) xrealloc (buffer, size);

      if (buffer != NULL)
	/* Try again. */
	snprintf (buffer, size, "value of %s is %s",
		  name, value);
    }
  /* The last call worked, return the string. */
  return buffer;
}

In practice, it is often easier just to use asprintf, below.

Attention: In versions of the GNU C Library prior to 2.1 the return value is the number of characters stored, not including the terminating null; unless there was not enough space in s to store the result in which case -1 is returned. This was changed in order to comply with the ISO C99 standard.